1. Field of the Invention
The present invention relates generally to the field of integrated optics. More specifically, the invention relates to integrated optical waveguide electro-optical modulators, that is devices based on the electro-optic effect in which optical beams propagate through optical waveguides integrated in an electro-optic substrate material, particularly optical intensity, i.e. amplitude, interferometric modulators of the Mach-Zehnder type. Still more particularly, the invention relates to a coplanar integrated optical waveguide electro-optical modulator, in which the electrodes necessary for applying a modulating electric field are arranged on a same substrate surface.
2. Technical Background
Integrated electro-optical devices, such as modulators and switches, are fabricated on substrates of electro-optic material. Among all the known substrate materials, lithium niobate (LiNbO3) is probably the most widely used because of the enhanced electro-optic properties thereof and the possibility of making low loss optical waveguides. Another known substrate material is for example lithium tantalate (LiTaO3).
Electro-optic materials show an electro-optic response, a second-order non-linear property which is characterized by a tensor. This tensor relates the polarization changes at optical frequencies (i.e., refractive index changes) of the material to low-frequency modulating electric fields, that is modulating electric fields at frequencies much lower than those of the optical fields. Phase and amplitude modulation of optical fields can be obtained by applying external electric fields, which modify the material refractive index via the electro-optic effect.
Overlooking, for simplicity, the tensorial nature of the electro-optic effect, the refractive index change xcex94n(xcfx89) at the optical frequency xcfx89 is proportional to the product of an electro-optic coefficient r and the modulating electric field Eo: xcex94n(xcfx89)xe2x88x9drxc2x7Eo.
In the case of a LiNbO3 crystal the electro-optic coefficient having the highest value is r33≈30 pm/V. The electro-optic coefficient r33 relates the refractive index change experienced by electromagnetic waves polarised along the c (also called z) crystal axis to the component of the modulating electric field along the same axis.
For this reason LiNbO3 crystal substrates are generally made available in z-cut slices, with the z crystal axis normal to the substrate surfaces of largest area, since this configuration is the one ensuring superior modulation performances even at relatively high modulation frequencies.
A Mach-Zehnder interferometric electro-optical modulator is a device capable of providing an electrically-induced amplitude modulation of an optical signal. In a Mach-Zehnder interferometric electro-optical modulator the voltage required to drive the modulator is reduced when the two optical modes propagating along the two interferometer arms experience changes of the refractive index having opposite sign. This is achieved by properly designing the electrode geometry, so that the component of the modulating electric field along the z axis has opposite signs (i.e. opposite orientations with respect to the z axis orientation) in the two interferometer arms. The resulting device is said to have a push-pull configuration.
FIGS. 1 to 4 show typical examples of push-pull Mach-Zehnder interferometric electro-optical modulators. Specifically, FIGS. 1 and 2 schematically show, respectively in top-plan and in cross-sectional views, a so-called coplanar waveguide (xe2x80x9cCPWxe2x80x9d) configuration. FIGS. 3 and 4 schematically show, again in top plan and in cross-section, a so-called double coplanar strip (xe2x80x9cCPSxe2x80x9d) configuration.
The electro-optic performances of devices based on the above cited configurations are extensively described in literature. For example, the performances of the CPW configuration is discussed in K. Noguchi et al., xe2x80x9840-Gbit/s Ti:LiNbO3 optical modulator with a two-stage electrodexe2x80x99, IEICE Trans. Electron., vol. E81-C, p. 316 (1998) and in K. Noguchi et al., xe2x80x98Millimiter-wave Ti:LiNbO3 optical modulatorsxe2x80x99, J. of Lightwave Tech., vol.16, p.615 (1998). The double CPS configuration is for instance described in U.S. Pat. No. 5,388,170.
Referring to FIGS. 1 and 2, in a z-cut LiNbO3 substrate 1 a Mach-Zehnder interferometer is integrated comprising an input optical waveguide 2 or input channel, a first Y-junction 3 for splitting an input optical signal propagating along the input waveguide 2 into two optical signals propagating along two generally parallel optical waveguides 41, 42 forming the interferometer arms, a second Y-junction 5, spaced apart from the first Y-junction, for combining the two optical signals into an output optical signal propagating along an output optical waveguide 6 or output channel. The waveguides 2, 41, 42 and 6 are formed by conventional techniques in correspondence of a surface 7 of the substrate 1 perpendicular to the z crystal axis. The substrate forms a single ferroelectric domain so that throughout the substrate the z crystal axis keeps a same orientation, for example the orientation shown by the arrow in FIG. 2.
In the region of the interferometer arms, a first metal electrode 8 is superimposed over the surface 7 above the waveguide 42 and extends for a section 421 thereof, a second metal electrode 9 is superimposed over the surface 7 above the waveguide 41 and extends for a section 411 thereof substantially corresponding to the section 421 of the waveguide 42, and a third metal electrode 10 is superimposed over the surface 7 and extends, laterally to the second electrode 9 and on the opposite side of the first electrode 8, for a segment substantially corresponding to the section 411 of waveguide 41. Conventionally, a buffer layer 11, typically of silicon dioxide (SiO2), is formed over the surface 7 for separating the metal electrodes 8, 9 and 10 from the optical fields in the waveguides 41, 42 so to avoid attenuation of said optical fields.
The electrodes 8, 9 and 10 are used for applying a modulating electric field useful for varying, by electro-optic effect, the refractive index in the two waveguides 41, 42. The electrodes 8 and 10 are electrically connected to a reference potential (ground), and are therefore called ground electrodes. The electrode 9 is electrically connected to a modulating potential V, and is called hot electrode. The shape and layout of the electrodes are properly designed so as to allow the operation of the device up to the microwave region of the spectrum of the modulating electric field. By applying a modulating electric field, the refractive index of the two waveguides 41, 42 undergoes opposite variations and the optical signals propagating along such waveguides correspondingly undergo opposite phase shifts (push-pull effect). An amplitude modulated output optical signal is thus obtained in waveguide 6, the amplitude depending on the overall phase shift.
The main disadvantage of the CPW configuration is the asymmetry of the structure, which gives rise to an asymmetry in the interaction between each optical mode propagating along the interferometer arms and the modulating electric field. Such an asymmetry causes different phase shifts in the two interferometer arms, thus inducing chirps in the phase of the amplitude modulated output optical field. This asymmetry is inherent to the device, since in order to have opposite phase shifts in the two interferometer arms the two waveguides must be placed one under the hot electrode and the other under the ground electrode. The efficiency of the phase shift induced on the optical mode propagating through a waveguide by the modulating electric field depends on the overlap between the modulating electric field and the optical mode, and is expressed by an overlap factor xcex93. The ratio of the overlap factors xcex93h in the waveguide 41 under the hot electrode 9 and xcex93g in the waveguide 42 under the ground electrode 8 is significantly high, reaching typical values of 6, so significantly different phase shifts take place in the two waveguides.
In the double CPS configuration of FIGS. 3 and 4, two hot electrodes 13 and 15 are provided, respectively superimposed over and extending for corresponding sections 421 and 411 of the waveguides 42 and 41. Two ground electrodes 12 and 14 are also provided, respectively adjacent a respective one of the hot electrodes 13 and 15. The two hot electrodes 13, 15 are electrically connected to modulating potentials +V, xe2x88x92V of opposite sign with respect to the ground potential. For this reason, this configuration is also called dual-drive.
In some cases, a third ground electrode, shown in dash and dot in the drawings and identified by 16, can be provided in between the hot electrodes 13 and 15, so that each of the latter extends between two ground electrodes. The resulting structure, ensuring a still higher symmetry, is said to have a double CPW configuration.
The double CPS configuration, as well as the double CPW one, does not suffer of the problem previously discussed in connection with the CPW configuration and is therefore referred to as chirp-free. However, the need for a dual drive significantly increases the complexity of the electronic circuits generating the driving potentials of opposite sign. This is a great disadvantage of the double CPS configuration.
JP 07-191352 discusses the problems of an optical waveguide device, such as a directional coupling optical switch, in which mutual exchange of wave energy between the waveguides takes place. This device, which is clearly different from an electro-optical modulator since in the latter no exchange of wave energy takes place between the waveguides, comprises a crystal substrate formed from a z-cut LiNbO3 crystal, in which two optical waveguides are formed adjacent and parallel to each other in the substrate surface. The device has a coupling region, that is the region of the substrate wherein the mutual exchange of wave energy between the waveguides takes place. Positive and negative electrodes are formed on the same substrate surface as the optical waveguides, with interposition of a buffer layer, and extend parallelly to each other in partial overlap with a respective waveguide. An electric field which curves toward the negative electrode from the positive electrode is generated, which has an action in approximately reverse directions, with respect to the z crystal axis, in the two waveguides.
According to JP 07-191352, in this configuration the direction of action of the electric field in the two optical waveguides is only approximately reverse, so there is a large loss of electric field action compared to a case of perfectly reverse directions. In addition, in order to ensure the most effective action of the electric field on both optical waveguides, fine position adjustment is necessary, by way of example edge sections of the electrodes are matched to the optimum position in the central region of the optical waveguide device so that the dense section of the electric field is concentrated on the optical waveguides. High-precision position adjustment of this kind on the minute optical waveguides is extremely difficult and hinders productivity improvements. Furthermore, because the positive and negative electrodes are formed in alignment on the same surface of the crystal substrate, a phenomenon (DC drift) is generated in which the operation voltage fluctuates due to the presence of the buffer layer between both electrodes, and this presents a significant problem in terms of actual application.
In that document the coplanar electrode arrangement is therefore excluded and a device is described which allegedly solves these problems. In the described device a pair of optical waveguides, formed on the surface of a z-cut lithium niobate crystal substrate, perform a mutual exchange of wave energy in a coupling area of the substrate. The z axis directions of the crystal, from which the optical waveguides are formed, are formed in mutually reverse directions, and opposing and parallel flat-plate positive and negative electrodes are arranged in the upper and lower surfaces of the crystal substrate. Based on this configuration, by the action of a linear, uniform and parallel electric field formed between the opposing flat-plate electrodes, an action in the respective reverse directions with respect to the z axis of the optical waveguides is effected.
The solution proposed by JP 07-191352 is based on the known property of ferroelectric materials that the signs of the non-linear optical coefficient (identified as d) and electro-optic coefficient (r) are related to the orientation of the ferroelectric domain, i.e. to the orientation of the spontaneous polarization of the crystal. When the orientation of the crystal spontaneous polarization is inverted both d and r change sign, being the manifestation of the second-order non-linear material response to, respectively high and low frequency, electromagnetic fields.
This phenomenon has been exploited in laser-diode-based second-harmonic generation (SHG) devices to achieve highly efficient quasi-phase-matched frequency conversion, as reported in M. Yamada et al., xe2x80x98First-order quasi-phase-matched LiNbO3 waveguide periodically poled by applying an external field for efficient blue second-harmonic generationxe2x80x99, Appl. Phys. Lett., vol. 62, p. 435 (1993).
In connection with electro-optical modulators, the effects of ferroelectric domain inversion have been exploited in U.S. Pat. No. 5,278,924 for obtaining an opto-electric modulator which compensates for phase velocity mismatches between optical modulation and an RF electric signal.
Both in SHG devices and in the device disclosed in U.S. Pat. No. 5,278,924 a suitable periodically inverted ferroelectric domain structure is requested and the optical modes propagate through many ferroelectric domain boundaries. The linear loss due to scattering and reflection at these boundaries is often below the detection level, in any case negligible when compared to any waveguide loss. In fact it is well known that differently oriented domains have the same linear dielectric properties (the refractive index is the same in differently oriented domains).
For example, in U.S. Pat. No. 5,278,924 an integrated optic Mach-Zehnder interferometer with an asymmetric coplanar waveguide travelling wave electrode is formed in a substrate which has a ferroelectric domain that has inverted regions and non-inverted regions. The inverted and non-inverted regions extend parallel to each other transversally to the interferometer arms in alternated succession along the arms. The optical signal in each interferometer arm passes through the inverted and non-inverted regions of the ferroelectric domain. Each transition between inverted and non-inverted regions changes the sign of the induced phase modulation of the optical signal. This compensates for 180xc2x0 phase difference between the modulation on the optical signal and the RF electric signal caused by the phase velocity mismatch between the RF and optical signals.
In still another context, in U.S. Pat. No. 5,267,336 the effects of ferroelectric domain inversion have been exploited to obtain an electric field sensor useful in detecting and measuring wideband transient electrical responses by means of an integrated optical waveguide Mach-Zehnder interferometer without any electrodes. This document addresses the drawbacks of conventional electro-optical sensors, in which an external electric field to be measured is picked up by an antenna and converted to a voltage which is then applied to appropriate electrodes positioned on or near the interferometer arms in such a way as to create electric fields in opposite directions in each of the two interferometer arms. According to this document, in a variety of applications that require the measurement of electric fields the presence of a metal electrode tends to disturb the electric field under measurement. In severe cases the close proximity of the electrodes could create arcing, thus creating a short circuit. The metal electrode also tends to limit the frequency response of the sensor due to the capacitive nature of the electrical circuit. This electrical circuit could also pose hazard in the presence of combustible or explosive materials. Therefore, according to this document, there is the desire to have electric field sensors that do not require any metal electrodes. According to this document, this is achieved by reversing the ferroelectric domains of one of the two interferometer arms, so that an external vertically directed electric field produce equal and opposite phase shifts in the two interferometer arms, leading to a total phase shift as experienced by the conventional electrode device.
The Applicant has observed that coplanar electrode arrangements are advantageous over non-coplanar counterparts, especially in high-frequency applications.
The Applicant has also found that exploiting the effect of sign reversal of the electro-optic coefficient to realise coplanar integrated electro-optical Mach-Zehnder type modulators in an electro-optic material substrate, for example by means of ferroelectric domain inversion in ferroelectric materials such as LiNbO3, so that at least in a modulating region of the device the sign of the electro-optic coefficient is different in one interferometer arm with respect to the other, new and advantageous modulator structures can be devised with respect to single domain electro-optic substrates. Such structures include for example a chirp-free coplanar waveguide modulator (where only one hot electrode is used) and a single-drive double coplanar strip modulator (where two hot electrodes are used).
According to one aspect of the invention, a coplanar integrated optical waveguide electro-optical modulator is provided, comprising:
a substrate of an electro-optic material;
at least two optical waveguides integrated in the substrate in correspondence of a surface thereof and defining a device modulation region therebetween, such waveguides disposed with respect to each other so as to substantially prevent the exchange of optical energy between said waveguides within the device modulation region, and
an electrode system arranged on said surface for applying a modulating electric field to the waveguides suitable for causing a modulation of a refractive index of the two waveguides in a device modulation region, characterized in that the waveguides are formed, for at least a section thereof in the device modulation region, in respective substrate regions each of which includes at least two successive segments having electro-optic coefficients with alternated sign, each successive segment being passed through by a respective one of the at least two waveguides, pairs of regions in the successive segments, substantially aligned in the direction transversal to the waveguides, having electro-optic coefficients of mutually opposite sign, so that a modulating electric field of same direction and orientation in the waveguide sections causes refractive index modulations of opposite sign in the waveguide sections.
In one embodiment, the waveguide sections and the respective substrate regions having electro-optic coefficients of opposite sign extend substantially for the whole device modulation region.
The electrode system may comprise at least two ground electrodes each one extending over said section of a respective waveguide, and at least a hot electrode extending between the ground electrodes. The electrodes thus form a coplanar waveguide electrode system.
Alternatively, the electrode system may comprise one hot electrode extending over said sections of the waveguides, and at least one ground electrode extending at the side of the hot electrode. Also in this case, the electrodes form a coplanar waveguide electrode system, but a drive voltage to be applied to the hot electrode can be reduced.
In this case, since the waveguides shall be spaced apart of a distance suitable to substantially prevent optical coupling therebetween in the modulation region, the hot electrode preferably comprises a wider portion having a width equal to or higher than said distance, and a narrower portion on the top of the wider portion.
In another embodiment, the electrode system comprises two hot electrodes, each one extending over said section of a respective waveguide, for receiving a same modulating voltage, and at least one ground electrode extending aside the hot electrodes, so as to form a double coplanar strip electrode system.
Preferably, the at least one ground electrode comprises two ground electrodes, each one extending aside a respective hot electrode on an side thereof opposite to the other hot electrode.
In a preferred embodiment, the coplanar integrated optical waveguide electro-optical modulator has an electrode system comprising an integrated power splitter for receiving an externally-generated modulating voltage and supplying it to the two hot electrodes.
Outside the modulation region the waveguides are optically connected by means of respective Y-junctions to an input waveguide and an output waveguide. The two hot electrodes may merge together at said Y-junctions and have extensions over the input and output waveguides.
Preferably, the ground electrodes extend aside said extensions so as to form, in correspondence of the input and output waveguides, coplanar waveguide electrode systems.
In case the substrate of electro-optic material is a z-cut substrate of ferroelectric material, that is a material having a spontaneous polarization, such as for example lithium niobate, the regions having electro-optic coefficients of opposite sign are regions having mutually oppositely oriented ferroelectric domains.
The substrate material may also be an x-cut substrate of ferroelectric material, particularly lithium niobate. In this case, the provision of opposite oriented ferroelectric domains allows to form a push-pull modulator by placing one hot electrode and one ground electrode.
According to a second aspect of the invention, an electro-optical modulator is provided comprising the coplanar integrated optical waveguide electro-optical modulator according to the first aspect of the invention, and an electrical drive element adapted for supplying to the electrode system a unipolar drive potential.
According to still another aspect of the invention, a transmission station for an optical communication system is provided, comprising at least an optical beam generation means for generating an optical beam and an electro-optical modulator according to the second aspect of the invention.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.